Sealant method of epoxy resin-clay composites

ABSTRACT

A clay-resin composition of a cured epoxy resin and a layered clay with the cured epoxy resin in the galleries of the clay by intercalation or exfoliation. The preferred epoxy resins are flexible and usually elastic because of the epoxy resin and/or curing agent which is used. The result is a composite which can have superior tensile strength and/or solvent resistance as compared to the cured epoxy resin without the clay or with the clay but without the intercalation or exfoliation. The flexible composites are particularly useful for seals and other thin layer applications.

GOVERNMENT RIGHTS

The invention described in this application was sponsored by theNational Science Foundation Contract CHE-92241023. The U.S. Governmenthas certain rights to this invention.

This is a divisional of copending application Ser. No. 08/498,350 filedon Jul. 5, 1995.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to novel flexible clay-resin compositecompositions containing a cured epoxy resin and to a method for theirpreparation. In particular, the present invention relates to the curedepoxy resin and clay composite containing the cured epoxy resin in thegalleries of the clay, such that the clay is intercalated or exfoliated.The composites are flexible and usually elastic.

(2) Description of Related Art

Layered or smectite clays are natural or synthetic layered oxides suchas bentonite, montmorillonite, hectorite, fluorohectorite, saponite,beidellite, nontronite, and related analogs. The layers are made up of acentral octahedral sheet, usually occupied by aluminum or magnesium,sandwiched between two sheets of tetrahedral silicon sites. Thesenegatively charged layers are approximately 10 Å thick, and areseparated by hydrated cations such as alkali or alkaline earth metalions.

Hybrid organic-inorganic composites can exhibit mechanical propertiessuperior to those of their separate components. To optimize theperformance properties of these materials, it is usually desirable todisperse the inorganic components in the organic matrix on a nanometerlength scale (Giannelis, E. P. JOM 44 28 (1992); Gleiter, H. Adv. Mater.4 474 (1992); and Novak, B. M., Adv. Mater. 5 422 (1993)). Smectiteclays and other layered inorganic materials that can be broken down intonanoscale building blocks (Pinnavaia, T. J., Science 220 365 (1983)) areuseful for the preparation of organic-inorganic nanocomposites.

In general, the polymer-clay composites can be divided into threecategories: conventional composites, intercalated nanocomposites, andexfoliated nanocomposites. In a conventional composite, the claytactoids exist in their original state of aggregated layers with nointercalation of the polymer matrix between the layers of the clay. Inan intercalated nanocomposite the insertion of polymer into the claylayer structure occurs in a crystallographically regular fashion,regardless of the clay-to-polymer ratio. An intercalated nanocompositenormally is interlayered by only a few molecular layers of polymer andthe properties of the composite typically resemble those of the ceramichost (Kato, C., et al., Clays Clay Miner, 27 129 (1979); Sugahara, Y.,et al., J. Ceram. Soc. Jpn. 100 413 (1992); Vaia, R. A., et al., Chem.Mater. 5 1694 (1993); and Messersmith, P. B., et al., Chem. Mater. 51064 (1993)). In contrast, in an exfoliated nanocomposite, theindividual 10-Å-thick clay layers are separated in a continuous polymermatrix by average distances that depend on loading. Usually, the claycontent of an exfoliated clay composite is much lower than that of anintercalated nanocomposite. Consequently, an exfoliated nanocompositehas a monolithic structure with properties related primarily to those ofthe starting polymer.

The exfoliation of smectite clays provides 10- Å-thick silicate layerswith high in-plane bond strength and aspect ratios comparable to thosefound for fiber-reinforced polymer composites. Exfoliated claynanocomposites formed between organocation exchanged montmorillonitesand thermoplastic nylon-6 have recently been described (Fukushima, Y.,et al., J. Inclusion Phenom. 5 473 (1987); Fukushima, Y., et al., ClayMiner 23 27 (1988); and Usuki, A., et al., J. Mater. Res. 8 1174 (1993);and WO 93/04117 and 93/04118 describing thermoplastic polymers). Clayexfoliation in the nylon-6 matrix gave rise to greatly improvedmechanical, thermal, and rheological properties, making possible newmaterials applications of this polymer (Usuki, A., et al., J. Mater.Res. 8 1179 (1993); and Kojima, Y., et al., J. Mater. Res. 8 1185(1993)).

The clays used for nanocomposite formation are ion-exchanged forms ofsmectite clays in which the Na⁺ and/or Ca²⁺ gallery cations of thepristine mineral have been replaced by organic onium ions. The oniumions may be protonated primary amines (RNH₃ ⁺), secondary amines (R₂ NH₂⁺), or they may be tertiary amines (R₃ NH⁺) or quaternary ammonium ions(R₄ N)⁺. The alkyl groups attached to nitrogen may be the same ordifferent, and the alkyl groups may be replaced in part by a benzylgroup (--CH₂ --C₆ H₅), a phenyl group (--C₆ H₅) or by benzyl and phenylgroups. The alkyl groups may also be functionalized, as protonated α,ω-amino acid with the general formula (H₃ N--(CH₂)--COOH)⁺. Phosphoniumions may be used in place of ammonium ions for the formation of claypolymer nanocomposites.

In some polymer-clay composite systems it is possible to continuouslyvary the amount of intercalated polymer between the clay layers from oneor a few monolayers of polymer chains to multiple layers of polymer. Insuch cases it is possible to prepare composites with properties thatvary from those typical of an "intercalated clay" nanocomposite to thosetypical of an "exfoliated clay" nanocomposite. The intercalated andexfoliated states of the clay may be distinguished based on the ratio ofthe observed gallery height (h_(o)) to the gallery height expected for alipid-like bilayer of onium ions in the gallery (h_(e)) The value ofh_(e) can be easily computer based on the van der Waals length of theonium ion (l). Thus, h_(e) =2l. The observed gallery height (h_(o)) canbe determined by subtracting the thickness of a smectite clay (9.6 Å)from the basal spacing obtained by X-ray diffraction measurement (d₀₀₁).As illustrated in FIGS. 1A to 1D, if h_(o) ≦h_(e), then the spacingbetween the clay layers allows for van der Waals interactions betweenthe onium ions on adjacent layers. These van der Waals interactionsbetween onium ion chains link adjacent clay layers. Under theseconditions, the intercalated polymer (polym.) resides in the galleryspace between the associated onium ions. The polymer-clay composites inthese cases can be regarded as an intercalated clay composite. However,if h_(o) >h_(e), as in FIG. 1E, then the adjacent layers of the clay areno longer linked through the onium ion interactions. However, the oniumion chains can reorient in the gallery to accommodate interaction of thepolymer. Thus, as illustrated in the example in FIGS. 1A to 1E as theamount of intercalated polymer is increased the initial lateral bilayerstructure of the organoclay in FIG. 1A has re-oriented to a paraffinstructure in FIG. 1B and to a perpendicular bilayer in FIG. 1C, alipid-like structure in FIG. 1D, and finally to an exfoliated structurein FIG. 1E. Consequently, the resulting state of the clay is bestdescribed as being exfoliated and the composite is an exfoliatedpolymer-clay composite. FIG. 1 illustrates these differences betweenintercalated and exfoliated polymer-clay composites in a system wherethe amount of interlayer polymer is varied continuously from one tomultiple layers of polymer chains.

According to the teachings of Lagaly, the onium ions in an organo claycan adopt various orientations depending on the clay layer chargedensity and the chain length of the onium ions. The observed orientationof onium ions in smectite clays include lateral monolayers, lateralbilayers, pseudotrimolecular layers, paraffin-like layers and lipid-likebilayers.

U.S. Pat. No. 4,889,885 to Usuki et al shows thermoplastic vinyl polymercomposites containing clay.

U.S. Pat. Nos. 3,432,370 to Bash et al; 3,511,725 to Stevens et al,3,847,726 to Becker et al and Canadian Patent No. 1,004,859 to Nelsonshow various compositions incorporating flexible epoxy resins. There arenumerous uses for these polymer matrices.

The problem has been to provide intercalated or exfoliated clay curedthermoset epoxy-resin composites. In general, it is difficult to obtainepoxy resins which are between the layers of the clay on a nanometerscale.

Epoxy resins tend to swell in the presence of organic solvents. Thisproblem is not corrected by merely mixing a clay into the epoxy resin.

OBJECTS

It is therefore an object of the present invention to provide a curedresin composition of a clay intercalated or exfoliated with a curedthermoset epoxy resin. Further, it is an object of the present inventionto provide preferred flexible and elastic cured thermoset epoxyresin-clay compositions with unique strength and solvent resistanceproperties. Further, it is an object of the present invention to providecomposites which are relatively easy and economical to prepare. Theseand other objects will become increasingly apparent by reference to thefollowing description and the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D illustrate intercalated claypolymer nanocomposites inwhich the gallery onium ion on adjacent clay layers link the layersthrough van der Waals interaction. The polymer in these examples occupyspace between the associated onium ions. FIG. 1E illustrates anexfoliated clay-polymer nanocomposite in which the onium ions onadjacent layers are separated by the guest polymer chains and the claylayers are no longer linked through van der Waals interactions betweenthe onium ions.

FIGS. 2A and 2B are graphs showing X-ray diffraction patterns of CH₃(CH₂)₇ NH₃ ⁺ -montmorillonite (FIG. 2A) and CH₃ (CH₂)₁₇ NH₃ ⁺-montmorillonite (FIG. 2B) in stoichiometric mixtures of epoxide resinand polyetheramine curing agent after reaction under the followingconditions: (a) 75° C., 10 min; (b) 75° C., 1 h; (c) 75° C., 3 h; (d)75° C., 3 h and 125° C., 1 h; (e) 75° C., 3 h and 125° C., 3 h. The clayloading in each case was 10 wt %. Samples a, b and c in both figurescontain intercalated clay particles as indicated by the d₀₀₁ diffractionlines, whereas samples d and e in both figures contain largelyexfoliated clay layers.

FIG. 3 is a TEM image of an exfoliated epoxy-clay composite containing20 wt % CH₃ (CH₂)₁₇ NH₃ ⁺ -montmorillonite showing the epoxy resin inthe layers of the clay.

FIGS. 4A and 4B are graphs showing dependence of tensile strength (FIG.4A) and modulus (FIG. 4B) of epoxy-organo clay composites on the chainlength of the clay-intercalated alkylammonium ions. The clay loading ineach case was 10 wt %. The dashed lines indicate the tensile strengthand modulus of the polymer in the absence of any clay.

FIGS. 5A and 5B are graphs showing dependence of tensile strength (FIG.5A) and modulus (FIG. 5B) on clay loading for exfoliated epoxy-CH₃(CH₂)₁₇ NH₃ ⁺ -montmorillonite nanocomposites.

FIGS. 6A and 6B are models for the fracture of a glassy (FIG. 6A) and arubbery polymer-clay (FIG. 6B) exfoliated nanocomposite with increasingstrains.

FIG. 7 is a graph showing the effect of alkylammonium ion chain lengthon the mechanical properties of epoxy-montmorillonite composites (2 wt%) using metaphenylene diamine as a curing agent to form a rigidpolymer-clay composite.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a flexible resin-clay compositecomposition which comprises: a flexible cured epoxy resin; andorgano-clay particles having layers and galleries between the layers,the galleries containing the cured epoxy resin and organic oniumcations, wherein the clay particles have a particle size about 0.02 and2 μm, wherein the ratio by weight of epoxy resin to clay is betweenabout approximately 200:1 and approximately 1:1, wherein the cured epoxypolymer composition has a glass transition temperature (Tg) betweenabout -100° C. and 100° C., and wherein the average separation betweenthe clay layers corresponds to a gallery height of 5 Å to 300 Å.

Further, the present invention relates to a method for the preparationof a flexible resin-clay composite composition which comprises: mixingparticles of an organo-clay having layers with galleries between thelayers which has been ion exchanged with organic onium cations with aflexible liquid epoxy resin and a curing agent which produces a curedepoxy resin; and curing the liquid epoxy resin with the curing agent toproduce the polymer-clay composite composition, wherein the galleriescontain the cured epoxy resin, wherein the cured epoxy polymercomposition has a glass transition temperature (Tg) between about -100°C. and 100° C., and wherein the average separation between the claylayers corresponds to a gallery height of 5 Å to 300 Å.

Finally, the present invention relates to a method for providing aflexible sealant in an apparatus operated at a particular temperature(T_(o)) the improvement which comprises: providing as the sealant aresin-clay composite composition which comprises: a flexible cured epoxyresin; and clay particles having layers and galleries between thelayers, the galleries containing the cured epoxy resin and organic oniumcations, wherein the clay particles have a particle size about 0.02 and2 μm, wherein the ratio by weight of epoxy resin to clay is betweenabout approximately 200:1 and approximately 1:1, wherein the cured epoxypolymer composition has a glass transition temperature (Tg) betweenabout -100° C. and 100° C., and wherein the average separation betweenthe clay layers corresponds to a gallery height of 5 Å to 300 Å.

The clay particles are preferably smectite clays with galleries of about3 to 30 Å in height before introduction of the epoxy resin. Suitablesmectite mineral clays are montmorillonite, hectorite, saponite,nontronite, or beidellite. Suitable synthetic derivatives of smectiteclays are fluorohectorite, laponite, taeniolite, tetrasilicic mica or amixed layer clay mica-montmorillonite. The layers are preferably about10 Å thick as discussed previously for smectite clays. The particleshave a particle size between about 200 and 20,000 Å and a ratio oflength to width in a range between about 2,000 to 1 and 20 to 1.

The clay is ion exchanged using ammonium salt, particularly analkylammonium halide such as a chloride, so that the cation is in thegallery of the clay. The alkyl group preferably contains between 3 and22 carbon atoms, which can be substituted with various non-carbon groupssuch as halogen (I, Cl, Br, F), nitrogen, oxygen, sulfur hydroxyl andthe like and can be straight chain or branched. Ion exchange isperformed as described in Pinnavaia, T. J., ACS Sym. Ser. 499 146(1992).

The epoxy resins are well known to those skilled in the art and aredescribed in Kirk-Othmer, John Wiley & Sons, 9 267-290 (1980). They areavailable from a variety of commercial sources including Shell Co.,Ciba, and The Dow Chemical.

Bisphenol A type EPON-828 (Shell Co.), is an epoxy resin precursor withthe bisphenol A structure and a molecular weight of 380, and has theformula: ##STR1## wherein n=0 (88%); n=1 (10%); n=2 (2%).

Bisphenol-A type, DER 331 (Dow Chemical Co., Midland, Mich.), is anepoxy polymer precursor and is an analog to Epon-828 having the formula:##STR2##

Bisphenol-F type, DER 354 (Dow Chemical Co.) is an epoxy polymerprecursor having the formula: ##STR3##

Novolac type, DER 43, DER 438 and DER 439 (Dow Chemical Co.) are epoxypolymer precursors having the formula: ##STR4## wherein n is betweenabout 0.2 and 1.8.

Epoxy polymer, DER 732 (Dow Chemical Co.) is an epoxy resin precursor ofthe general formula: ##STR5## There are numerous other types of epoxypolymer precursors which are suitable and which are well known to thoseskilled in the art.

Amine curing agents are used to cure the epoxy resin precursors into anepoxy resin. The most preferred curing agents are polyoxypropylene di-or triamines which are sold as JEFFAMINES, Huntsman Chemical Company,Austin, Tex. Most preferred are the polyoxypropylene diamines (D-series)of the formula: ##STR6## wherein x is between about 4 and 40.

The preferred diamines when used as curing agents for the epoxy resinprecursors produce a glass transition temperature of less than ambienttemperatures (25° C.) and preferably less than 0° C. As a result, whencured to a pristine epoxy resin without any filler, the resins areflexible when x is between about 4 and 40 in the polyoxypropylenediamine, the cured epoxy resin is also elastic.

The T series JEFFAMINES can be used. These are ##STR7## wherein x+y+zbetween about 4 and 120.

Various other epoxy resin curing agents, such as anhydrides and amides,can be used, so long as they do not interfere with the curing action ofthe primary ammonium cations in the clay. The amide curing agents arefor instance ##STR8## where X is between about 5 and 15.

The organic onium cations in the clay act to catalyze the epoxy resin insitu in the galleries. As a result the clay is intercalated orexfoliated with the cured epoxy resin. The ion-exchanged organic oniumcation intercalated clay is mixed with the epoxy resin precursorpreferably using shear mixing. Usually the mixture is degassed with avacuum. The mixture is cured at between about 50° C. to 150° C.depending upon the epoxy resin precursor (monomer or prepolymer) and thecuring agent.

When the resin-clay composites of the present invention are flexible,they are very strong in comparison to the pristine epoxy resin. Theflexible composites of the present invention are particularly useful assealants and flexible adhesives. They are strong, exhibiting arelatively high tensile strength. The compositions of the presentinvention can be used for: surface coatings, particularly decorativecoatings; protective coatings; casting and encapsulation; construction,particularly seamless floors, sand-filled flooring, decorativeaggregate, polymer concrete, trowel coatings, and wood consolidation;reinforced composites, particularly for propeller and impeller blades,boats, filament-wound tanks and piping; and adhesives. Other uses wherea relatively thin flexible layer is needed are for instance in thedampening of interfaces between vibrating surfaces.

The following are illustrative Examples 3 to 28, 30 and 31 showing theuse of various clay epoxy polymer precursors, amine and amide curingagents. Examples 1 and 2 are Comparative Examples. Example 29 showsabsorption data. Example 32 shows a rigid epoxy-clay polymer compositeprepared from meta phenylene diamine as a curing agent.

COMPARATIVE EXAMPLE 1

A pristine epoxy polymer with a sub-ambient glass transition temperature(-40° C.) was prepared by crosslinking Epon-828 epoxy resin (Shell) andJEFFAMINE D2000 (Huntsman Chemicals, Austin, Tex.) polyetheramine curingagent. Equivalent amounts of the epoxide resin (27.5 wt %) and thepolyetheramine (72.5 wt %) were mixed at 75° C. for 30 minutes. Theepoxide-amine complex was then outgassed in vacuum for 10 minutes andtransferred into an aluminum mold for curing at 75° C. for 3 hours andthen at 125° C. for an additional 3 hours. The pristine epoxy matrix wasnamed Example 1 (E1). E1 has tensile strength of 0.6 MPa and tensilemodulus 2.8 MPa. ##STR9##

COMPARATIVE EXAMPLE 2

A conventional clay-epoxy composite (E2) was prepared from naturallyoccurring Na-montmorillonite from Wyoming. The Na-montmorillonite waspurified and the portion of 40˜50 μm was used to prepare the composite.Equivalent amounts of the epoxide resin (27.5 wt %) and thepolyetheramine (72.5 wt %) were mixed at 75° C./ for 30 minutes. 10 wt %of the clay was added to the epoxide-amine mixture and stirred foranother 30 minutes. The clay-epoxide-amine complex was then outgassed invacuum for 10 minutes and transferred into an aluminum mold for curingat 75° C. for 3 hours and then at 125° C./ for an additional 3 hours. E2is a phase-segregated composite. Tensile strength and modulus of thecomposite are 1.2 and 3.5 MPa, respectively. The phase segregation ofthe clay in the polymer matrix indicates little interaction between theinorganic clay and the polymer matrix. XRD analysis shows that the clayretains its basal spacing after composite formation.

EXAMPLES E3-E8

An improvement of affinity between resin matrix and clay is essential toprepare useful polymer-clay composite. Akylammonium exchanged clays(organoclays) enhance phase affinity and nanocomposite formation.Examples 3-7 demonstrate the relationship between the chain length ofthe onium exchange ion and the extent of epoxy resin intercalation (XRD)and the tensile properties of the resin-clay composite. The organoclayswere obtained by ion exchange reaction of Na-montmorillonite. The cationexchange reaction was carried out by mixing 500 ml of 0.05Malkylammonium chloride ethanol:water (1:1) solution and 2.0 g of clay at70°˜75° C. for 24 hours. The exchanged clays were washed withethanol:water (1:1) several times until no chloride was detected with1.0M AgNO₃ solution and then air dried. Finally, the clays were groundand the particle size fraction of 40˜50 μm was collected. Thehydrophobicity of clay gallery surface of an organoclay is controlled bythe chain length of the gallery alkylammonium cations. The extent ofepoxy resin intercalation into the clay is dependent on thehydrophobicity of the interlayer gallery regions. Alkylammonium cationswith different alkyl chain length (carbon number 4, 8, 10, 12, 16 and18) were used to prepare organo montmorillonites. Epoxy resin wasEpon-828 from Shell Co. Curing agent is JEFFAMINE D2000. Equivalentamounts of the epoxy resin (27.5 wt %) and the polyetheramine (72.5 wt%) were mixed at 75° C. for 30 minutes. 10 wt % of the organoclays wasadded to the epoxide-amine mixture and stirred for another 30 minutes.The clay-epoxide-amine complex was then outgassed in vacuum for 10minutes and transferred into an aluminum mold for curing at 75° C. for 3hours and then at 125° C. for an additional 3 hours. The compositesamples were labeled Examples E3, E4, E5, E6, E7, and E8, representingalkylammonium chain length, C4, C8, C10, C12, C16, and C18 respectively.X-ray diffraction results for the epoxy resin-clay composites indicatethat the clays with alkyl chain length ≧10 (E5, E6 and E7, and E8) theabsence of a d₀₀₁ diffraction. Therefore, C10-C18 chain alkylammoniumexchanged montmorillonite clays were exfoliated in the cured epoxy resinmatrix. Clay layer separation in sample E5, E6, E7 and E8 was observedfrom TEM images obtained from ultrathin sections. In comparison, the E3(C4) and E4 (C8) alkylammonium exchanged montmorillonite clays,exhibited d₀₀₁ X-ray diffraction in the cured composites. Tensilestrengths, moduli and clay properties for examples E1, E2, E3, E4, E5,E6, E7 and E8 are listed in Table 1, indicating the mechanicalperformance was improved significantly with clay exfoliation in thematrix.

                  TABLE 1                                                         ______________________________________                                        Gallery Chain Length Effect on Nanocomposite Formation and                    Properties.                                                                                          Clay                                                                  Initial Gallery                                                               Clay    Height  Length                                                Onium   Gallery in Com- of          Mod-                                      ion,    Height  posite  Onium Strength                                                                            ulus                               Example                                                                              (n).sup.a                                                                             (Å) (Å) Ion (Å)                                                                         (MPa) (MPa)                              ______________________________________                                        E1     na      na      na      na    0.6   2.8                                E2     na      na      na      na    1.2   3.5                                E3     4       3.6     ≧7.sup.b                                                                       10.0  1.3   8.1                                E4     8       4.2     ≧8.sup.b                                                                       15.1  2.9   9.0                                E5     10      4.2      80-110.sup.C                                                                         17.6  3.2   12.2                               E6     12      6.0      80-100.sup.C                                                                         20.2  3.5   13.8                               E7     16      8.0     100-130.sup.c                                                                         25.3  3.6   13.9                               E8     18      8.4     100-150.sup.c                                                                         27.8  3.6   14.5                               ______________________________________                                         .sup.a: n of gallery ions (CH.sub.3 (CH.sub.2).sub.n-1 NH.sub.3.sup.+ ;       .sup.b: Obtained from XRD;                                                    .sup.c: Obtained from TEM.                                                    MPa = pascals ÷ 1,000,000.                                           

The tensile strengths and moduli of the epoxy resin-clay nanocompositeshave been measured according to the ASTM method 3039 using a UniversalTest System (UTS). The relationship between the alkylammonium cationchain length of the organoclay and the mechanical properties of thecomposites is illustrated in FIG. 4 for loadings of 10 wt % CH₃(CH₂)_(n-1) NH₃ ⁺ -montmorillonite. The presence of the organoclaysubstantially increases both the tensile strength and the modulusrelative to the pristine polymer. The mechanical properties increasewith increasing clay exfoliation in the order CH₃ (CH₂)₇ ⁻ NH₃ ⁺ -<CH₃(CH₂)₁₁ NH₃ ⁺ -<CH₃ (CH₂)₁₇ NH₃ ⁺ - montmorillonite. It is noteworthythat the strain at break for all of the epoxy resin-clay composites isessentially the same as the pristine matrix, suggesting that theexfoliated clay particles do not disrupt matrix continuity.

The X-ray diffraction patterns of the epoxy resin-clay compositescontaining CH₃ (CH₂)₇ NH₃ ⁺ --and CH₃ (CH₂)₁₇₋ NH₃ ³⁰ -montmorilloniteare shown in FIG. 1A. These diffraction patterns reveal the change inclay basal spacing that occurs in the epoxy curing process. It isnoteworthy that CH₃ (CH₂)₇ NH₃ ⁺ - and CH₃ (CH₂)₁₇ NH₃ ⁺-montmorillonites respond differently to the epoxide-poly(etheramine)reaction mixture. For CH₃ (CH₂)₇ NH₃ ⁺ -montmorillonite (cf. FIG. 2A),the initial basal spacing at 15.2 Å is retained throughout the curingprocess, but significant broadening and reduction of scatteringintensity occur for this reflection at 125° C. For CH₃ (CH₂)₁₇ NH₃ ⁺-montmorillonite (cf. FIG. 2B), the basal spacing increases withreaction time and temperature. A new diffraction line with basal spacingat 54 Å appears after curing at 75° C. for 3 hours, while the intensityof the original clay diffraction line decreases. With further curing at125° C., the clay diffraction lines are too broad to be distinguishable.Therefore, for CH₃ (CH₂)₁₇ NH₃ ⁺ -montmorillonite, an exfoliatednanocomposite is achieved. In contrast, CH₃ (CH₂)₇ NH₃ ⁺-montmorillonite is only partially exfoliated in the polymer matrix.

The formation of exfoliated clay nanocomposites is dependent on thenature of the alkylammonium-exchanged clays. Longer linear alkyl chainsfacilitate the formation of the nanocomposite. Heating the CH₃ (CH₂)₁₇NH₃ ⁺ -montmorillonite system at 75° C. causes the epoxide and amine tomigrate into the clay galleries and form an intermediate with a 54Åbasal spacing. Upon additional heating, further polymerization iscatalyzed by the acidic primary ammonium ion (Kamon, T., et al., InEpoxy Resins and Composites I.V; Dusek, K., Ed. Springer-Verlag: Berlin;Advances in Polymer Science, 80 177 (1986); Barton, J. M., Advances inPolymer Science, 72 120 (1985); and May, C. A., Ed. Epoxy Resins, 2nded; Marcel Dekker: New York (1988)) and more epoxide and amine enter thegalleries, leading to the formation of an exfoliated nanocomposite.Thus, the exfoliation of the clay is caused by intragallery resinformation. In the case of CH₃ (CH₂)₇ NH₃ ⁺ -montmorillonite, thehydrophobicity of the galleries is relatively low, and the amount ofintercalated epoxide and amine is insufficient to achieve exfoliation.Therefore, only a portion of the clay is delaminated, as evidenced bythe broadening and decreased scattering intensity of the 15.2 Åreflection, and the remainder of the clay retains its original basalspacing. Consequently, the final product in the case of CH₃ (CH₂)₇ NH₃ ⁺-montmorillonite is a mixture of exfoliated and conventional claycomposites.

A typical TEM image of the epoxy-exfoliated clay nanocompositecontaining CH₃ (CH₂)₁₇ NH₃ ⁺ -montmorillonite is shown in FIG. 3. Thedark lines are the cross sections of the 10-Å-thick silicate layers. Aface--face layer morphology is retained, but the layers are irregularlyseparated by ˜80-150 Å of resin. This clay particle morphology iscorrelated with the absence of Bragg X-ray scattering. The TEM imagesand XRD patterns of the epoxy composite formed with CH₃ ¹ (CH₂)₁₁₋ NH₃ ⁺-montmorillonite were essentially the same as those for the CH₃ (CH₂)₁₇NH₃ ⁺ -montmorillonite system.

Reinforcement of the epoxy-clay nanocomposites also is dependent on clayloading. As shown in FIG. 5A and 5B, the tensile strength and modulusfor the CH₃ (CH₂)₁₇ NH₃ ⁺ -montmorillonite system increases nearlylinearly with clay loading. More than a 10-fold increase in strength andmodulus is realized by the addition of only 15 wt % (˜7.5 vol %) of theexfoliated organoclay.

For the low-T_(g) epoxide-amine system of the present invention, thereinforcement provided by the exfoliated clay is much more significant.Owing to the increased elasticity of the matrix above the T_(g), theimprovement in reinforcement may be due in large part to sheardeformation and stress transfer to the platelet particles. In addition,platelet alignment under strain may also contribute to the improvedperformance of clays exfoliated in a rubbery matrix as compared to aglassy matrix. The most significant difference between glassy andrubbery resins is the elongation upon stress. The rubbery epoxy matrixused in these Examples exhibits 40-60% elongation at break, whereas forthe glassy epoxy resin matrix it was reported previously that it wasonly 5-8%.

As illustrated in FIGS. 6A and 6B, the clay platelets in the cured resinare capable of being aligned in the direction of the matrix surface.When strain is applied in the direction parallel to the surface, theclay layers are aligned further. This strain-induced alignment of thelayers enhance the ability of the particles to function as the fibers infiber-reinforced plastics. Propagation of fracture across the resinmatrix containing aligned silicate layers is energy consuming, and thetensile strength and modulus are reinforced. In a glassy matrix, clayparticle alignment upon applied strain is minimal and blocking of thefracture by the exfoliated clay is less efficient.

EXAMPLES E9-E13

Examples 9-13 illustrate the relationship between clay charge density,the degree of resin intercalation and tensile properties of compositesformed from different 2:1 clays having the same gallery onium ions,namely CH₃ (CH₂)₁₇ NH₃ ⁺. Clay layer charge density is an importantfactor in determining the clay exfoliation in the polymer matrix.Several smectite clays such as hectorite (Hect), montmorillonite(Wyoming and Arizona), and fluorohectorite (Fhect) were used to prepareepoxy-clay composites. For comparison, non-smectite 2:1 clays such asvermiculite (Verm) and rectorite (Rect) also were used to prepare epoxyresin-clay composites. The preparation method is essentially the same asdescribed in Example E3-E8. C18 alkylammonium cation was used as galleryions. The composite samples were labeled E9, E10, E11, E12, and E13 forcomposites containing hectorite, montmorillonite from Arizona,fluorohectorite, rectorite, and vermiculite, respectively. Clay galleryheights, clay layer density and tensile properties are listed in Table2. The Example E12 contains rectorite, a regularly interstratified clayunlike normal smectite clays has a layer thickness of 20 Å rather than10 521 . Thus, the exfoliation of rectorite in epoxy matrix provides 20Å-thick silicate layers dispersed uniformly in the matrix. Naturalrectorite was purified and ion-exchanged to CH₃ (CH₂)₁₇ MH₃ ⁺-rectorite. Comparing E12 and E8, it was noticed that at the same clayloading, rectorite composite has a comparable strength and modulus asmontmorillonite composite, even though the number of clay layers in therectorite composite is nearly half of those montmorillonite composite.Therefore, the 20 Å-thick rectorite clay layers are more effective thanthe 10 Å-thick clay layer as reinforcement particle in a flexiblematrix.

                  TABLE 2                                                         ______________________________________                                        Composite Formation and Properties with Different Layer Charge                Density Clays (E8-E13), clay loading is 10 wt %.                                                            Clay                                                                   Initial                                                                              Gallery                                                      Layer     Clay   Height                                                       charge    Gallery                                                                              in Com-                                                      density (e.sup.- /                                                                      Height posite                                                                              Strength                                                                            Modulus                             Ex.  Clay    O.sub.20 (OH).sub.4)                                                                    (Å)                                                                              (Å)                                                                             (MPa) (MPa)                               ______________________________________                                        E8   Mont-   0.76      24.5.sup.a                                                                           80-110.sup.b                                                                        3.6   14.5                                     Wy                                                                       E9   Hect    0.66      25.7.sup.a                                                                           80-110.sup.b                                                                        3.0   14.5                                E10  Mont-   1.2       25.0.sup.a                                                                           80-110.sup.b                                                                        3.7   14.0                                     Az                                                                       E11  Fhect   1.4       24.1.sup.a                                                                           95.sup.a,b                                                                          2.5   15.1                                E12  Rect    0.7       25.1.sup.a                                                                           80-120.sup.b                                                                        3.2   13.1                                E13  Verm    1.6       25.3.sup.a                                                                           45.sup.a                                                                            1.8   9.0                                 ______________________________________                                         .sup.a: Obtained from XRD;                                                    .sup.b: Obtained from TEM.                                               

EXAMPLES E14-E17

These examples show the dependence of clay reinforcement on clayloading. Epoxy resin-clay nanocomposites were prepared according to thepreparation method described in the E3-E8 with different clay loadingsprepared. The examples with clay contents of 2, 5, 15 and 23 wt % werelabeled E14, E15, E16 and E17, respectively. X-ray diffraction resultsindicate that they are exfoliated clay nanocomposites. Tensileproperties of the examples, along with the reference epoxy resin fromExample 1 are given in FIG. 5A and 5B.

The results in FIG. 5A and 5B clearly show the reinforcing effect of theexfoliated clay layers in the resin matrix. Similar results also wereobserved for exfoliated clay nanocomposites containing other smectiteclays.

EXAMPLES E18-E21

These examples demonstrate the relationship between layer separation andtensile properties of resin-clay nanocomposites. Exfoliated epoxy-claynanocomposites were prepared by using in situ gallery intragallerycatalyzed polymerization. Intercalated nanocomposites were also preparedwithout intragallery catalyzed polymerization. The gallery cationacidity plays a very important role in determining the intragalleryepoxy-amine polymerization rates and in controlling the clay exfoliationin the epoxy-amine thermosetting process. Quaternary alkylammonium ionsuch as CH₃ (CH₂)₁₇ N(CH₃)₃ ⁺ (C 183M) was used to prepare organoclays.X-ray diffraction results indicated that the composites have distinctd₀₀₁ diffraction at 42 Å, corresponding to a 32 Å gallery height.Therefore, they are typical intercalated nanocomposites. The examplesare labeled E18, E19, E20, and E21 for the composites containing CH₃(CH₂)₁₇ N(CH₃)₃ ⁺ -montmorillonite at 2, 5, 10 and 15 wt % clay loading.Tensile properties of the intercalated nanocomposites are listed inTable 3. For comparison, the exfoliated nanocomposites prepared withsimilar chain length primary alkylammonium exchanged clays (E8, E14-16)are listed as well.

                                      TABLE 3                                     __________________________________________________________________________    Comparison of Tensile Properties of Intercalated and Exfoliated               Epoxy-Montmorillonite                                                         Clay Nanocomposites.                                                                    Initial Clay                                                                        Clay Gallery   Clay                                                                              Tensile                                                                           Tensile                                    Gallery                                                                             Gallery                                                                             Height in      Loading                                                                           Strength                                                                          Modulus                                Sample                                                                            Onium Ions                                                                          Height (Å).sup.a                                                                Composite(Å)                                                                     Composite Type                                                                        (wt %)                                                                            (MPa)                                                                             (MPa)                                  __________________________________________________________________________    E18 Q     12.5  32.0.sup.a                                                                           intercalated                                                                          2   1.3 6.5                                    E14 P     8.4   100-220.sup.b                                                                        exfoliated                                                                            2   0.9 5.9                                    E19 Q     12.5  32.0.sup.a                                                                           intercalated                                                                          5   1.7 6.9                                    E15 P     8.4   80-160.sup.b                                                                         exfoliated                                                                            5   1.6 7.6                                    E20 Q     12.5  32.0.sup.a                                                                           intercalated                                                                          10  2.6 12.4                                   E8  P     8.4   80-110.sup.b                                                                         exfoliated                                                                            10  3.6 14.5                                   E21 Q     12.5  32.0.sup.a                                                                           intercalated                                                                          15  3.0 15.2                                   E16 P     8.4   80-110.sup.b                                                                         exfoliated                                                                            15  7.4 24.1                                   __________________________________________________________________________     P: Primary ammonium ion CH.sub.3 (CH.sub.2).sub.17 NH.sub.3 +; Q:             Quaternary ammonium ion CH.sub.3 (CH.sub.2).sub.17 N(CH.sub.3).sub.3 +;       .sup.a Obtained from XRD; .sup.b Obtained from TEM.                      

The mechanical property data in Table 3 demonstrate that the reinforcingeffect for intercalated and exfoliated nanocomposites are comparable atlow clay loading up to 5 wt % and similar in magnitude to the pristineresin. However, at higher clay loadings in the range 10 and 15 wt %, thereinforcing effect for the exfoliated nanocomposites is much larger thanthe intercalated nanocomposites. Thus, at clay loading above about 5 wt%, the extent of clay layer separation determines the performanceproperties of the epoxy resin-clay nanocomposites. Similar effects wereobserved for fluorohectorite and other 2:1 clay systems.

EXAMPLES 22-24

These examples demonstrate the relationships between reinforcementproperties and the extent of clay layer exfoliation. CH₃ (CH₂)₃ NH₃ ⁺,CH₃ (CH₂)₇ NH₃ ⁺, and CH₃ (CH₂)₁₇ NH₃ ⁺ exchanged montmorillonites areused to prepare epoxy resin-clay nanocomposites. The epoxy resin andcuring agent are identical to the composition described in Examples 3, 4and 8. The curing condition is modified to hot-mold-casting, which meansthat transfer of the degassed epoxy-clay composition to a preheated-(125° C.) mold and continuance of the curing at 125° C. for 6h. Thehot-mold-casting method promote the intercalation of the epoxy monomerto the clay galleries in the clay and clay exfoliation, especially forthe shorter chain alkylammonium exchanged clays. In Table 4, epoxy-claycomposites prepared by the hot-mold-casting method are compared with theones with the same composition but prepared by the conventional castingmethod.

                  TABLE 4                                                         ______________________________________                                        Comparison of Tensile Properties of Epoxy-clay Composites Containing          Alkylammonium Exchanged Montmorillonite (10 wt %) Prepared under              Different Curing Conditions.                                                                              Clay                                                                    Initial                                                                             Gallery                                                                 Clay  Height                                                                  Gallery                                                                             in Com-                                                Onium   Curing   Height                                                                              posite Strength                                                                            Modulus                              Ex.  ion, (n).sup.a                                                                        Condition                                                                              (Å)                                                                             (Å)                                                                              (MPa) (MPa)                                ______________________________________                                        E1   None    CC       --    --     0.6   2.8                                  E3   4       CC       3.6   ≧7.sup.b                                                                      1.3   8.1                                  E22  4       HMC      3.6   ≧10                                                                           2.1   12                                   E4   8       CC       4.2   ≧8.sup.b                                                                      2.9   9.0                                  E23  8       HMC      4.2   ≧100.sup.c                                                                    4.7   16                                   E8   18      CC       8.4   100-150.sup.c                                                                        3.6   14.5                                 E24  18      HMC      8.4   100-150.sup.c                                                                        3.8   14.6                                 ______________________________________                                         .sup.a: n of gallery ions CH.sub.3 (CH.sub.2).sub.n-1 NH.sub.3.sup.+ ;        .sup.b: Obtained from XRD;                                                    .sup.c: Obtained from TEM.                                                    CC: Conventional casting;                                                     HMC: hotmold-casting.                                                    

Tensile properties of these composites are compared in Table 4. For thecomposites containing CH₃ (CH₂)₃ NH₃ ⁺ -, as well as CH₃ (CH₂)₇ NH₃ ⁺-montmorillonite, a great improvement in mechanical properties isachieved by using the hot-mold-casting method. However, for thecomposite containing CH₃ (CH₂)₁₇ NH₃ ⁺ -montmorillonite, which is easilyexfoliated under both normal and hot-mold-casting conditions, no furthermechanical improvement is observed.

EXAMPLES E25-E28

Epoxy-clay nanocomposites were prepared by using other epoxy resins fromDOW Chemical Co. and JEFFAMINE D2000 containing CH₃ (CH₂)₁₇ NH₃ ⁺-montmorillonite. The basic structures of the DOW epoxy resin are givenas follows: ##STR10##

The epoxy-clay nanocomposites prepared from above-mentioned epoxy resinwith 10 wt % CH₃ (CH₂)₁₇ NH₃ ⁺ -montmorillonite have exfoliatedstructures. Their mechanical properties compared to those of thepristine resin matrices are superior and comparable to the resultsobtained from Epon 828-D2000 systems of Examples 1 to 24.

EXAMPLE 29

Solvent adsorption experiments for Examples E1, E2, E3 and E8.

Solvent absorption data were obtained using a mass differentialtechnique. Samples of a known weight were immersed in toluene at roomtemperature and reweighed periodically after blotting to remove excesssolvent. The difference between two weighings corresponded to tolueneuptake. Samples were weighted until constant weight at which point itwas assumed that they had reached equilibrium. The relative rate oftoluene adsorption and total uptake at equilibrium of Examples E1, E2,E3 and E8 are listed in Table 5.

                  TABLE 5                                                         ______________________________________                                        Relative Toluene Adsorption Rate and Total Uptake of Epoxy-Clay               Nanocomposites.                                                                                    Relative Initial                                         Nanocomposites                                                                          Type of Clay                                                                             Adsorption  Relative Total                               Example   Composite  Rate (%)    Uptake (%)                                   ______________________________________                                        E1        na         100         100                                          E2        conventional                                                                             95          100                                          E3        intercalated                                                                             84          93                                           E8        exfoliated 42          51                                           ______________________________________                                    

The toluene adsorption results exhibit a substantial decrease in solventuptake for the exfoliated epoxy-clay nanocomposite E8. The exfoliated 10Å-thick clay layer in the matrix may contribute to the decrease of therate and amount of the solvent uptake. Whereas, for the intercalatednanocomposite and the conventional composite, the clay exists astactoids, the effect of clay in decreasing solvent adsorption is notsignificant. Similar experiments with hexane adsorption were carried outand comparable results were obtained.

The adsorbed organic molecules can be removed by air-drying orvacuum-drying. During the drying process, the pristine polymer matrix(E1), conventional composite (E2), and intercalated nanocomposite (E3)crack, whereas, the exfoliated nanocomposite (E8) retains its integrity.Therefore, for the exfoliated nanocomposite, the adsorption-desorptionof organic molecules occurs with structural reversibility. The abilityof the nanocomposite to return to its pristine structural state upondesorption of dissolved organic solvent greatly extends their use assealants and gaskets which may contact organic substances frequently.

EXAMPLE 30

Exfoliated epoxy-clay nanocomposites can also be obtained by replacingthe D2000 curing agent in Examples E8, E9, E10, E11, and E12 withglyceryl poly(oxypropylene)triamine curing agents of the type ##STR11##and x+y+z=4˜120. These curing agents are commercially available underthe trademark JEFFAMINE 403, T703, T3000 and T5000.

EXAMPLE 31

The curing of epoxy resin is not limited to amine curing agent.Replacement of the amine curing agents in Examples E8, E9, E10, E11 andE12 with polyamide of the type ##STR12## and x=5˜15, also affordsnanocomposite materials with greatly improved mechanical properties.These polyamide curing agents are commercially available under thetradename VERSAMID 140, 125 and 100, available from Henkel, Ambler, Pa.

EXAMPLE 32

Comparative epoxy resin-exfoliated clay nanocomposites formed fromEpon-828 resin and m-phenylenediamine as the curing agent were preparedusing long-chain alkylammonium-exchanged smectite clays. Due to the highglass transition temperature (˜150° C.) of the epoxy matrix, thecomposites were in a glassy state at room temperature. The results areshown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Mechanical Properties of Epoxy-Clay Nanocomposites formed with                CH.sub.3 (CH.sub.2).sub.15 NH.sub.3.sup.+  Clays                              Clay Samples  Failure Strength                                                                          Tensile Modulus                                     (1 wt %)      (MPa)       (GPa)                                               ______________________________________                                        Laponite      83 ± 2   1.59 ± 0.04                                      Hectorite     94 ± 2   1.65 ± 0.03                                      Montmorillonite                                                                             92 ± 5   1.49 ± 0.03                                      Fluorohectorite                                                                             73 ± 3   1.55 ± 0.06                                      ______________________________________                                    

It was found that the improvement over the admixtures with the clay werenot as pronounced as with the polyoxypropylene diamines. This can beseen from FIG. 7.

The inventors described this Example 32 in publications by Lan et al,Proceedings of the ACS, PMSE 71 527-528 (published August 1994) and Lanet al, Chemistry of Materials 6 2216-2219 (December 1994). A derivativework based on the inventors' work was published in Chemical Materials 61719-1725 (December 1994).

It is intended that the foregoing description be only illustrative ofthe present invention and that the present invention be limited only bythe hereinafter appended claims.

We claim:
 1. In a method for providing a flexible sealant in an apparatus the improvement which comprises: providing as the sealant a resin-clay composite composition which comprises:(a) a cured epoxy resin; and (b) a smectite clay having layers with the cured epoxy resin separating the layers, the galleries containing the cured epoxy resin and organic onium cations wherein the onium cations are protonated unsubstituted alkyl ammonium cations containing 3 to 22 carbon atoms in the alkyl ammonium cations, wherein the composition contains between about 5 and 50 weight percent of the clay, and wherein the average separation between the clay layers corresponds to a gallery height of 7 Å to 300 Å.
 2. The composition of claim 1 wherein an epoxy resin precursor is cured with a diamine as the curing agent to provide the cured epoxy resin.
 3. The method claim 1 wherein a diglycidyl ether of bisphenol A monomer is cured with a polyoxypropylene diamine as the curing agent to provide the cured epoxy resin.
 4. The method of claim 1 wherein a Novolac epoxy prepolymer is cured with a polyoxypropylene diamine to provide the cured epoxy resin.
 5. The method of claim 1 wherein a diglycidyl ether of bisphenol F monomer is cured with a polyoxypropylene diamine to provide the cured epoxy resin.
 6. The method of claim 1 wherein a diglycidyl polyalkylene ether epoxy prepolymer is cured with a polyoxypropylene diamine to provide the cured epoxy resin.
 7. The method of claim 1 wherein an epoxy monomer or prepolymer is cured with a polyoxypropylene diamine having a formula: ##STR13## wherein x is between about 4 and 40 as the curing agent to provide the cured epoxy resin.
 8. The method of claim 1 wherein the epoxy resin is a diglycidyl ether of bisphenol A monomer cured with a polyoxypropylene diamine having a formula: ##STR14## wherein x is between about 4 and 40 as the curing agent to provide the cured epoxy resin.
 9. The method of claim 1 wherein the epoxy resin is derived from a diglycidyl ether of bisphenol F monomer cured with a polyoxypropylene diamine having a formula: ##STR15## wherein x is between about 4 and
 40. 10. The method of claim 1 wherein the epoxy resin is derived from a Novolac polymer cured with a polyoxypropylene diamine having a formula: ##STR16## wherein x is between about 4 and
 40. 11. The method of claim 1 wherein the epoxy resin is a diglycidyl polyalkylene ether epoxy resin cured with a polyoxypropylene diamine having a formula: ##STR17## wherein x is between about 4 and
 40. 12. The method of claim 1 wherein the clay is selected from the group consisting of smectite clay minerals, montmorillonite, hectorite, saponite, nontronite or beidellite, and synthetic derivatives, fluorohectorite, laponite, taeniolite or tetrasilicic mica.
 13. The method of claim 1 wherein the clay is a synthetic mixed layer clay mica-montmorillonite. 